1. Technical Field
This invention relates to the technological art of mechanically finishing metal surfaces and to the art of thermally depositing metals onto substrates, and more particularly to performing such technological arts with an aluminum substrate such as an aluminum engine block.
2. Discussion of the Prior Art
The standard surface preparation process used in the thermal spray industry is that of grit blasting metal substrates employing a media, such as silica, sand, alumina, chilled iron or garnet, to produce a roughened surface topography characterized by a dimpled or hammered effect, usually free of oxides. Typically about 0.1 lb. per minute of grit is propelled through a nozzle at inlet pressures of as high as 1.8 ksi. in a gas (N.sub.2 or air) medium. Outlet velocities may be supersonic and the volume of material removed will vary as particle velocity is raised. The blasted surface must then be cleaned to remove any foreign contaminants or residual grit or oxides. In spite of such cleaning, there is always the problem of grit or oxide entrapment in the prepared surface and grit contamination of the part and surrounding machinery as a result of such blasting. Grit blasting can be damaging to the final product and to the manufacturing process equipment resulting in increased maintenance costs and reduced productivity; on-line collection, containment and disposition of a grit media is a difficult and environmentally unfriendly process.
Sometimes rough threading, by a single point machining tool, is utilized to create a roughened surface; often, this rough threading is combined with grit blasting to achieve the final roughened surface. The problem with rough threading, as a surface preparation for thermal spray, is that it does not provide an adequate level of bond strength. Rough threading produces non-intersectingintersecting grooves and each groove is smooth and shallow, resulting from the use of a fixed cutting tool having a semi-circular cutting tip or radius. The grooves are regularly spaced, which inhibits bond strength of a coating applied thereto.
Aluminum substrates, particularly those to be used in very severe operating conditions, such as experienced in a cylinder chamber of a internal combustion engine, present a more challenging problem for adhesion of the coating to the prepared surface. The tenacious formation of aluminum oxide on any exposed aluminum surface may inhibit chemical or mechanical bonding of the superimposed metallic coating irrespective of the type of thermal spraying employed.
It is, accordingly, an object of this invention to provide an enhanced mechanical/chemical bond between a thermally sprayed metallic coating and an aluminum substrate without the disadvantage of high cost of surface preparation or the disadvantages of grit contamination, oxide residual, and grit disposal associated with prior art technologies.